No, I'm not really up to speed on the differences... have you looked at the specs for each to compare? I'm thinking they would be very similar with minor differences at best.
I would not update MC6 to MC6t... I would put that money towards a true upgrade to the Sonar X3 line. You get more inputs and depending on which X3 product you buy, you get lots of bells and whistles and other functions that MC does not, and will never have in it.